The present invention relates to a receiving apparatus which receives an OFDM (Orthogonal Frequency Division Multiplex) modulated signal and, in particular, relates to a receiving apparatus which suppresses the influence due to noise components contained in the receiving signal to improve a carrier to noise ratio (CN ratio).
In the communication field such as the mobile communication etc., the OFDM modulation system is applied which multiplexes the transmission information by the frequency division to make it possible to transmit the information with high quality and at a high frequency utilizing efficiency.
Conventionally, the arrangement shown in FIG. 11 is known as an OFDM receiving apparatus which receives the modulated signal according to the OFDM modulation system. In the figure, an antenna 1 receives an OFDM modulated radio signal. The high frequency signal thus received is frequency converted into an intermediate frequency signal IF by a front end including a high frequency amplifier 2, a mixer 3 and a local oscillator 10. Further, the intermediate frequency signal IF is passed through a band pass filter 4 and amplified by an intermediate frequency amplifier 5 to a level capable of being signal processed. The signal thus amplified is supplied to a demodulating section formed by an analog-to-digital (A/D) converter 6, a demodulator 7, an OFDM demodulator 8 and a frame synchronous detector 12.
An oscillator 11 is controlled by a system controller 13 in a manner that it generates predetermined timing clocks and supplies these clocks to the A/D converter 6, the demodulator 7 and the OFDM demodulator 8.
The A/D converter 6 converts the aforesaid intermediate frequency signal IF into a digital signal on the basis of the timing clocks from the oscillator 11 and supplies the digital signal to the demodulator 7. The demodulator 7 detects the intermediate frequency signal IF in synchronism with the timing clocks from the oscillator 11 to generate an in-phase component signal (I component signal) and a quadrature component signal (Q component signal) as a base band signal.
The frame synchronous detector 12 detects the frame synchronous signal from the signal IF having been A/D converted by the A/D converter 6 to make the OFDM demodulator 8 perform the demodulating process on the basis of the timing clocks from the oscillator 11. The demodulator 7 supplies the I component signal and the Q component signal to the OFDM demodulator 8.
Then, the OFDM demodulator 8 subjects the I component signal and the Q component signal to the fast Fourier transformation (FFT) process at the predetermined timings synchronous with the frame synchronous signal and further to the differential demodulation process thereby to generate a demodulated signal and outputs the signal.
Although not shown, the demodulated signal is subjected to the error correction process and the decoding process, and the decoded signal is outputted to a signal processing circuit 9. The signal processing circuit 9 subjects the compressed signal contained in the decoded signal to the expansion process etc. to convert into an original audio band signal etc. and then outputs to a speaker etc.
In the aforesaid conventional OFDM receiving apparatus, in order to suppress the influence due to external noises or internal noises mixed in the intermediate frequency signal IF generated by the mixer 3 thereby to generate the demodulated signal with high quality, the method of removing such noises by using the band pass filter 4 is employed.
However, in order to remove the noise component from the OFDM modulated signal which is formed by multiplexing multiplicity of sub-carriers by the frequency division thereby to distinguish only the signal component of the respective sub-carrier frequencies, it is necessary to provide a band pass filter having a narrow pass frequency band and a high Q at every sub-carrier frequency. Such provision of the band pass filters results in the technical difficulty and is a cause for preventing the improvement of noise suppressing effect.
For example, when the frequency interval of the respective sub-carriers (the frequency between the adjacent channels) is a small value of about. 1 KHz, a band pass filter with quite steep frequency discrimination characteristics is required. Thus, since the noise suppressing effect depends on the variance etc. of the frequency characteristics of the band pass filter 4, there arises a problem that sufficient noise suppression effect can not be obtained and the cost becomes expensive.
Accordingly, the present invention has been made so as to obviate the aforesaid problems of the prior art, and an object of the present invention is to provide a receiving apparatus which can suppress the influence due to noises contained in a receiving signal thereby to improve a carrier to noise ratio (CN ratio).
In order to attain the aforesaid object, the invention is arranged in a manner that in a receiving apparatus for receiving an OFDM signal which is transmitted in a form of a frame having a no-signal portion and a data portion subjected to OFDM modulation by a plurality of sub-carriers, the receiving apparatus includes
frequency conversion section for converting a tuning signal into an intermediate frequency signal;
noise detection section for detecting a noise component contained in the no-signal portion as a noise component contained in a signal of the sub-carrier frequencies; and
frequency deviation section for deviating the intermediate frequency within an interval of the sub-carrier frequencies in accordance with the noise component contained in the no-signal portion detected by the noise detection section.
According to such an arrangement, the frequency of the tuning signal (reference frequency signal) used at the time of frequency-converting the received OFDM modulation signal into the intermediate signal is finely adjusted within the interval of the sub-carrier frequencies. This fine adjustment of the frequency is performed in accordance with the noise component of the no-signal portion contained in the OFDM signal, for example. In other words, the frequency of the tuning signal is feedback-controlled in accordance with the noise component. When the frequency conversion is performed based on the tuning signal thus finely adjusted, the sub-carriers in the intermediate frequency signal are separated from the noise component on the frequency axis. Further, when the intermediate frequency signal where the sub-carriers and the noise component are separated is subjected to the Fourier transformation for the demodulation, the noise component can be suppressed. As a result, the CN ratio can be improved and the signal can be received with high quality.
Further, the invention is arranged in a manner that the receiving apparatus further includes error rate detection section for detecting an error rate at a time of decoding data contained in the data portion, wherein
the frequency deviation section deviates the intermediate frequency within the interval of the sub-carrier frequencies in accordance with the noise component contained in the no-signal portion detected by the noise detection section and the error rate thus detected.
According to such an arrangement, when the error rate of the signal to be actually demodulated and decoded becomes large, the frequency of the tuning signal is finely adjusted in accordance with the error rate. Like the aforesaid manner, when the frequency conversion is performed based on the tuning signal thus finely adjusted, the sub-carriers in the intermediate frequency signal are separated from the noise component on the frequency axis. Further, when the intermediate frequency signal where the sub-carriers and the noise component are separated is subjected to the Fourier transformation for the demodulation, the noise component can be suppressed. As a result, the CN ratio can be improved and the signal can be received with high quality.